Introduction:I think everyone have heard about genes - such a nucleotide-chain, which encodes a product (protein). But sometimes (in myosis period) there can happen a nucleotide aberration (translocation, isomerisation e.c.t) or even deletion, of course there are and other factors. But many mutations happens de novo(new spontaneous mutations). And that is the main cause of different genetic illness. Till 1980 gene therapy was like a surrealistic dream for people, but now it is widely used in treating such disease as cancer (especially melanoma ), SCID (severe combined immune deficiency), hemophilia...So the principle of gene therapy is to change the "bad" fragment of RNA with a "good" one and here begins the problem. It is how to deliver RNA piece and locate "the missing piece of puzzle". Now scientists use modified viruses (vectors) to deliver and change the cell genome. (note.: viruses are not a cell form of live, which can only replicate by interfering its genome into cell and force cell to make many copies of itself) but in our case viruses only change the broken piece. So, in that topic I would like to discuss the + and - of gene therapy.